Why does render resolution affect output quality?
There are 2 reasons:


the down-scaling can have a sharpening effect on its own - depends on the algorithm used
the higher resolution image contains more traced samples per the same area = less noise. The less noise the less blurry image. Let's say single pixel in original size received 100 samples, the same area in double the resolution would receive 400 samples.


So the apples-to-apples comparison should be rendering the original resolution with 4x the sample count and down-scaling the higher resolution with a method that does not add any sharpening.